This invention relates to digitally encoded voice signals. In particular, it is a method and means for automatic selection of one of a plurality of digitally encoded bit streams that has been decoded.
Radio communications among fixed stations, mobile units and portable units cannot be greatly localized in direction without losing the possibility of coverage as the mobile or portable units change locations. Such communications are therefore easily intercepted by an eavesdropper. if security of communications is important to the user, it can be achieved by a system such as the Motorola Digital Voice Protection System. This is a system that converts an electrical analog of a voice signal into a digital bit stream by continuously-variable-slope delta (CVSD) modulation. The digital bit stream is then scrambled or encrypted by a system to which only the sender and authorized receivers are given keys. When such a system is in use, the eavesdropper may detect the signals, but he receives no more than a pseudorandom signal resembling noise because he lacks means to decrypt the signal.
The user of a two-way radio having a digital encryption system such as the Motorola Digital Voice Protection System needs first to be able to listen to clear signals in his channel and to be able to respond with a clear transmission. The term "clear" is here used to denote analog modulation that is typically frequency modulation in the channels of interest. The user also needs to be able to detect and respond to signals that are encoded in binary form and then scrambled. Detection of the presence of digital signals can be carried out by a system such as that of U.S. Pat. No. 3,995,225, which is assigned to the assignee of the present invention. That patent is incorporated here by reference as if set forth fully herein.
If receiver circuitry has detected the presence of a binary signal, it is next necessary to submit it to an unscrambling process. This requires that the user have available a decrypting system containing the key with which the message was encrypted. If he does and if he applies the message to such a system, the decrypted signal will be a binary signal that represents delta modulation by an audio signal. The delta modulation may be continuously variable-slope delta (CVSD) modulation. If the user has not had the proper key, the result of subjecting the signal to a decryption process with the wrong key will be to produce a signal with noise-like properties. A decrypted signal can be distinguished from a signal that has not been decrypted by a circuit such as the one of U.S. Pat. No. 4,197,502, assigned to the assignee of the present invention. That patent is incorporated here by reference as if set forth fully herein. The circuit of that patent provides a signal which can be used to mute a receiver if a digital signal has not been decoded and to enable the receiver if the signal has been decoded.
The foregoing combination of operations provides a thorough measure of voice protection in the scrambled mode, while allowing the user to communicate in the clear with other users who either do not have or do not choose to use a scrambled mode. It is sometimes desirable in such systems to maintain different levels of security. A supervisor, for example, might wish to deliver a scrambled message to one user or a set of users without communicating with another set of users on the same channel. This has been accomplished by allowing the supervisory transceiver to have a plurality of scramblers and to select among them manually. The same feature has been extended to mobile and portable units in which manual selection has been made of one of a plurality of unscramblers. Manual selection is the method of choice with a supervisory transmitter or other unit that originates calls. It is a disadvantage, however, for the unit receiving calls in that a muting system of the type described above will leave the called unit unaware that it is being called in a scrambled mode to which it has not selected the key.
One solution to this problem is to call a remote unit in the clear and identify a particular key that is to be used to unscramble an ensuing message. This represents a compromise in security in that the clear transmission alerts eavesdroppers to the fact of traffic between the units even if they are later unable to decrypt the message. Another means of establishing communication is to reserve one particular key as a supervisory channel and to use that key to instruct users to select another key for incoming messages. This requires that the user select the channel for the incoming message and then remember to return his key selection to the supervisory channel. It is evident that each of these methods represents a level of undesired complexity of operation.
It is an object of the present invention to provide a method and means of automatic selection of a decrypted signal.
It is a further object of the present invention to provide automatic selection of one encrypted signal that has been subjected to a plurality of decryption schemes, including one that decrypts the signal.
Other objects will become apparent in the course of a detailed description of the invention.